Polish
Don't register shutdown hook from embedded context

Change `EmbeddedWebApplicationContext` to no longer automatically call
`registerShutdownHook()`. Shutdown hooks must now be registered by the
caller (for users of SpringApplication this will happen automatically).

Fixes gh-314
Consistent javadoc "code" block formatting

Update javadoc to consistently use the "code" style with <pre> blocks.
Also include the latest spring-javadoc.css formatting file.

Fixes gh-332